This is a wrapper around File::Flock that starts a subprocess and
does the locking in the subprocess with File::Flock. The purpose of
this is to handle operating systems (eg: Solaris) that do not retain
locks across a call to fork().

The sub-process for this is created with fork() when
File::Flock::Subprocess is compiled. Ive tried to minimize the
side-effects calling fork() by doing calling it early and by using
POSIX::_exit() to quit but it is still worth being aware of. I suggest
loading File::Flock::Subprocess early.